As devices grow smaller and/or more complex, the electrical components that make up the devices must grow smaller. For example, an integrated circuit may include a large number of logic gates, which in turn are made up of even more transistors. If it is desired to decrease the size of the integrated circuit, the logic gates and the transistors that make up the logic gates must be made smaller. Alternatively, if it is desired to provide additional functions in the integrated circuit, more logic gates (hence transistors) are necessary. If the logic gates and transistors are to take up the same amount of space (i.e., the device is to remain the same size), the logic gates and transistors must again be made smaller.
As transistors are made smaller, the threshold voltages of the transistors are subject to greater variation. That is, the threshold voltage of any one transistor may fall within a wider range of values. Because transistors are often used in pairs, it is desirable for the threshold voltages of each transistor of a pair to be very close to each other, if not the same. If the threshold voltages of the two transistors in a pair are sufficiently different, the device (i.e., logic gate) of which they are components may simply fail to operate. In a device consisting largely of logic components, the failure of one or more of these logic components can cause the device to malfunction, or possibly to cease to function altogether.
The problem of mismatched threshold voltages may be illustrated with respect to a circuit as shown in FIG. 1. The circuit of FIG. 1 is a sense amplifier. Sense amplifier 100 has a pair of bit lines 111 and 112, and a pair of data lines 121 and 122. Sense amplifier 100 is designed to detect slight differences between signals on bit lines 111 and 112, and to amplify these differences. The amplified signals are produced on data lines 121 and 122. An enable signal is input to the circuit on lines 131 and 132. In this embodiment, when the enable signal goes high, the difference between the signals on bit lines 111 and 112 is amplified and provided on data lines 121 and 122.
When sense amplifier 100 is not enabled, nodes 131 and 132 of FIG. 1 are pre-charged to Vdd-Vth, where Vth is the threshold voltage of the respective one of NMOS transistors 141 and 142. In other words, node 131 is pre-charged to Vdd-Vth (of transistor 141), and node 132 is pre-charged to Vdd-Vth (of transistor 142). These pre-charge voltages are equalized somewhat by PMOS transistor 151. When Vdd is low and the pre-charge voltages at nodes 131 and 132 are near the threshold voltage of transistor 151, however, transistor 151 switches very weakly. Consequently, the equalization of the pre-charge voltages between nodes 131 and 132 may not be very good, and there may be a difference between these voltages. This voltage difference may be enough to counteract the voltage difference that would otherwise be detected between bit lines 111 and 112, causing sense amplifier 100 to malfunction.
Further, when sense amplifier 100 is operated at high frequencies, it may be very difficult to pre-charge nodes 131 and 132 to Vdd-Vth. This is a result of several factors. First, the pre-charge time of data lines 121 and 122 becomes short as the voltages of nodes 131 and 132 approach Vdd-Vth (where Vth is the threshold voltage for the respective one of transistors 141 and 142). Also, transistors 141 and 142 are turned off as the voltages at nodes 131 and 132 approach Vdd-Vth. When sense amplifier 100 is activated before nodes 131 and 132 are pre-charged to Vdd-Vth, transistors 161 and 160 operate in a linear region rather than a saturation region, and the current difference that is generated between these transistors becomes smaller. Because the current difference between transistors 161 and 162 is smaller, any mismatch between the threshold voltages of transistors 141 and 142 has a greater impact on the operation of sense amplifier 100. As a result, sense amplifier 100 must be operated more slowly.